1. Field of the Invention
The present invention relates to methods and apparatus for displaying an image by employing a laser beam or beams.
2. Description of the Prior Art and Related Information
High resolution displays have a variety of applications, including computer monitors, HDTV and simulators. In such applications, the primary considerations are resolution, maximum viewable area, cost and reliability. Although a number of approaches have been employed including CRT displays, rear projection and front projection displays, plasma displays and LCDs, none of these have been able to satisfactorily provide all the above desirable characteristics. In other display applications, such as control panel displays, and vehicle and aircraft on-board displays, resolution is of less importance than brightness, compact size and reliability.
Although lasers potentially can provide many advantages for displays of both types noted above, laser based displays have not been widely employed. This is due in large part to limitations in the laser scanning engines available. One conventional approach to scanning a laser beam employs a rotating mirror to scan the laser beam in a linear direction as the mirror rotates. Typically, the mirror is configured in a polygon shape with each side corresponding to one scan length of the laser beam in the linear direction.
An example of such a rotating polygon laser beam scanner is illustrated in FIG. 1. The prior art laser beam scanning apparatus shown in FIG. 1 employs a polygon shaped mirror 1 which receives a laser beam provided by laser 2 and deflects the laser beam in a scanning direction X as the polygon 1 rotates. It will be readily appreciated from inspection of the geometry of FIG. 1 that such a rotating polygon system has the ability to scan the laser beam through a maximum angle of 180.degree. with a scan line duration determined by the rotational speed of the polygon divided by N, where N is the number of polygon sides. Also, it will be appreciated that for large N the scan angle may be significantly reduced below 180.degree.. Thus, for the eight sided polygon configured as illustrated in FIG. 1, the laser beam is scanned through an angle of about 90.degree. with the duration of each scan line being 1/8 the period for one rotation of the polygon.
The laser scanning apparatus illustrated in FIG. 1 has the advantage of being quite simple, and is suitable for some applications. Nonetheless, this conventional laser scanning apparatus is not suitable for high resolution displays since the inherent limitations of such apparatus make it difficult to simultaneously achieve a high degree of resolution, high scanning speed and a large scanning angle. More specifically, a high degree of resolution requires a relatively large polygon with few sides. That is, if the laser beam is to provide accurate information as it is scanned along the scan direction, modulation of the laser beam as it traverses the surface of the polygon side must unambiguously provide discrete points in the scan direction. Thus, each side of the polygon must increase with the beam diameter and the number of discrete scan points (n). Therefore, high resolution, corresponding to a very large number (n) of discrete scan points, in general requires large polygon sides. This limitation is particularly significant where the scanned beam target surface is located close to the polygon mirror. Also, as noted above, the scan angle is reduced as the number of polygon sides is increased. Therefore, high resolution and high scan angle require a large polygon with relatively few sides.
The requirements of a large polygon with few sides, however, mitigate against a high scan rate and thus severely restricts resolution and/or refresh rate of a display based on such a laser beam scanning apparatus. As indicated above, scanning speed is directly related to the number of polygon sides. Therefore, a polygon with few sides requires very high speed rotation to achieve high scanning speed. Rotating a large polygon at high speed creates mechanical problems, however. In particular, high speed rotation introduces vibrations, stress on the moving parts, and reduced accuracy in the registration of the mirror relative to the laser beam. These factors collectively limit the rotational speed of the mirror, and hence the beam scan rate.
As noted above, another category of display application of increasing importance requires relatively small but robust displays having good brightness and acceptable resolution for graphics, such as maps, and text. Such displays have significant applications in automobiles and other vehicles. In such applications, a laser based display has potential advantages due to its brightness. However, once again, the existing laser beam scanning apparatus are not well suited. In particular, the optical path of the laser beam is quite short in such applications due to the compact space available for the display. This requires the size of the rotating polygon to be increased. However, mechanical instability is associated with large rotating polygons and is a serious detriment for such applications, where reliability is critical.
Accordingly, it will be appreciated that a need thus presently exists for an improved laser beam display apparatus.